Rod guides for centralizing sucker rods within production tubing are known in the prior art. As shown in FIG. 1, a pumping unit has attached thereto a sucker rod 10. (FIG. 1 was copied from U.S. Pat. No. 5,180,289 to Wenholz et al. and assigned to Baker Hughes Incorporated). At the bottom end of the sucker rod 10 is a reciprocating pump (not shown). As the pumping unit moves the sucker rod 10 down, the barrel of the reciprocating pump fills with the production fluid to be produced. Conversely, as the pumping unit moves the sucker rod up, a valve in the reciprocating pump shuts and the production fluid in the pump barrel is lifted, displacing production fluid above it and forcing one pump-barrel's worth of production fluid out of the hole.
The sucker rod must extend from the pumping unit all the way down to the reciprocating pump, which may be several thousand feet below the surface. Consequently, the sucker rod is subjected to a variety of stresses: compression, tension, torsion, and bending. Further, the sucker rod can "wobble" within the production tubing. This problem of "wobble" has been solved by the installation of rod guides on the sucker rod to centralize the sucker rod within the production tubing thereby controlling rod and tubing wear.
A prior art sucker rod guide includes a body that is molded in intimate contact with the sucker rod. The body has simultaneously molded therewith a plurality of "fins" or "blades" that extend radially from the body. As used herein, the term "fin" or "blade" refers to the molded portion of the rod guide that extends from the body to guidingly contact the interior surface of production tubing.
Known prior art rod guides include a convex contour of the body between blades. The location at which a blade meets the body thus defines an interior corner or root. It has been found that this interior corner is a weak spot in the rod guide and is inordinately more likely to fail than other regions of the rod guide. Thus, there remains a need for a rod guide without a convex portion of the body between the blades. In fact, this portion of the body preferably defines a strictly concave contour between blades.
In operation, the sucker rod is immersed in production fluid. As the sucker rod moves up and down to pump fluid from down hole, the rod guide provides resistance to the movement of the sucker rod due to hydraulic action of the fluid through and around the rod guide. Known rod guides have provided an extended length of the rod guide in order to give an adequate erodable volume of rod guide material while providing sufficient area through the rod guide for fluid flow. Known rod guides also present a flat (though slanted) aspect of the face of each blade to the fluid, both on the upstroke and the downstroke of the sucker rod. Such a flat aspect develops further resistance to fluid flow through the rod guide. Finally, the flat aspect of the face of each blade develops turbulent fluid flow behind the rod guide, further inhibiting movement of the rod guide up and down within the production tubing.
Thus, there remains a need for a rod guide that has an adequate volume of erodable material while maximizing cross sectional area for production fluid flow. Such a rod guide should present a smooth, contoured "knife-blade" aspect for the face of each fin of the rod guide to minimize resistance to the movement of the sucker rod and to eliminate turbulent fluid flow behind each fin.
As noted above, rod guides are subject to a variety of stresses. One such stress on rod guides results from a bending moment that has been shown to be one significant source of rod guide failure. One reason for this is that rod guides are primarily made of plastic that is molded directly upon a sucker rod. The material from which the rod guide is molded must conform to a standard from the National Association of Corrosion Engineers (NACE), Std. TM-01-87-Hydrocarbon Mixture With 500 psi gas consisting of 87.5% CO.sub.2 and 12.5% H.sub.2 S. This standard dictates a material which is resistant to temperature and chemicals (e.g., H.sub.2 S, certain salts, etc.) and such a material is inherently brittle. Rod guides are commonly made of rieton, nylon, polyurethane, or the like.
To provide a predictable site for rod guide failure, Positive Action Tool Co. of Dallas action produced a rod guide known as "double-plus." "Double-plus" provided two pairs of fins, offset circumferentially from one another by 90.degree.. However, such an arrangement apparently does nothing to reduce the likelihood of such a failure, it simply predetermines where such a failure will occur. Also, such a design presents the same resistance to fluid flow and, in fact, appears to make undesirable turbulent flow more likely.
Thus, there remains a need for a rod guide that is more robust to bending moment without sacrificing any of the other important features previously noted.